The present invention relates to the art of electrical amplifiers. It finds particular application in conjunction with driving gradient coils in magnetic resonance imaging (MRI) scanners, and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also amenable to other like applications.
In MRI, a substantially uniform temporally constant main magnetic field, B.sub.0, is generated within an examination region. The main magnetic field polarizes the nuclear spin system of a subject being imaged within the examination region. Magnetic resonance is excited in dipoles which align with the magnetic field by transmitting radio frequency (RF) excitation signals into the examination region. Specifically, RF pulses transmitted via an RF coil assembly tip the dipoles out of alignment with the main magnetic field and cause a macroscopic magnetic moment vector to precess around an axis parallel to the main magnetic field. The precessing magnetic moment, in turn, generates a corresponding RF magnetic resonance signal as it relaxes and returns to its former state of alignment with the main magnetic field. The RF magnetic resonance signal is received by the RF coil assembly, and from received signals, an image representation is reconstructed for display on a human viewable display.
The appropriate frequency for exciting resonance in selected dipoles is governed by the Larmor equation. That is to say, the precession frequency of a dipole in a magnetic field, and hence the appropriate frequency for exciting resonance in that dipole, is a product of the gyromagnetic ratio .gamma. of the dipole and the strength of the magnetic field. In a 1.5 T magnetic field, hydrogen (.sup.1 H) dipoles have a resonance frequency of approximately 64 MHZ. Generally in MRI, the hydrogen species is excited because of its abundance and because it yields a strong MR signal.
To spatially or otherwise encode the magnetic resonance, MRI systems employ gradient coil assemblies that are typically pulsed with electrical current pulses to produce magnetic gradients across the main magnetic field in the vicinity of the imaging region. For different imaging experiments, various waveforms are used to generate the desired gradient pulse sequence. The effect of the gradient pulse is to locally modify the frequency and/or phase of the nuclear magnetic resonance (NMR) signal through a change in magnitude of a z component of the main magnetic field, B.sub.o. With stronger gradient strength, higher resolution is realized in an acquired image. It is therefore desirable to have an amplifier which can deliver high levels of current and/or voltage to the resistive and/or inductive loads represented by the gradient coils. Moreover, as different imaging experiments call for different gradient pulse sequences produced by current pulses having various waveforms, it is advantageous for the amplifier be able to arbitrarily generate any desired waveform.
Early amplifiers used to supply current to gradient coils were analogue, both vacuum and solid state, and hence dissipated considerable heat. Later solid state four quadrant amplifiers were made using pulse width modulation (PWM) techniques.
Previously implemented amplifiers include linear, PWM (see, for example, U.S. Pat. No. 5,519,601 to Close et al.), hybrid speed-up schemes (see, for example, Mueller et al., "A GTO Speed-Up Inverter for Fast-Scan Magnetic Resonance Imaging," Conf. Proc. IEEE, (1992), pp. 479-486), and stepped modulation. However, while linear amplifiers provide low noise, high fidelity, and large control bandwidths, they are intrinsically inefficient driving an inductive load. Therefore, they use large amounts of silicon.
A four quadrant PWM scheme (i.e., a full bridge configuration) using metal-oxide-semiconductor field-effect transistor (MOSFET) or insulated gate bipolar transistor (IGBT) type devices can provide large amounts of power more efficiently than linear designs, but it takes special care to achieve low noise or high fidelity. Additionally, this PWM design is fundamentally limited on voltage unless a large number of devices are combined into an array.
Speed-up type amplifiers use large storage capacitors and a simple switch in combination with a standard linear amplifier serving as a regulator. This provides high voltages, but only limited control of the waveform transitions.
With stepped modulation, stacks of multiple four quadrant PWM amplifiers are used to achieve the full voltage range desired for MRI. This is cumbersome and in effect costly, particularly considering that only half the silicon in each full bridge is utilized at a time.
The present invention contemplates a new and improved amplifier for driving MRI gradient coils which overcomes the above-referenced problems and others.